DESCRIPTION: Pathological angiogenesis is an essential component of tumor growth, development, and metastasis for which few therapeutic options exist. Although a great deal about angiogenesis has been learned over the past several decades, it remains unclear how integral membrane receptors cooperate with one another to influence cellular signaling in response to extracellular cues. Two important families of receptors in angiogenesis, the Ties and Integrins, respond to the extracellular environment via outside-in and, in the case of Integrins, inside- out signaling. Recently, it was reported that the endothelial specific tyrosine kinase receptor, Tie2, forms complexes with two of the endothelial Integrins heterodimers, ?5?1 and ?v?3, providing a convenient clarification for the integration of extracellular stimuli. However, our preliminary studies suggest that Integrins bind to both Tie1 and Tie2 on the cellular membrane, and that binding occurs strictly through the extracellular domains. To elucidate the biological role of these interactions, biochemical and biophysical methods including co-immunoprecipitation, confocal microscopy, and FRET will be used to follow receptor/Integrin association in response to the Tie2 ligands Angiopoietin-1 and -2 as well as the Integrin ligands fibronectin, and vitronectin. Furthermore, structural determination either through x-ray crystallography or cryo-electron microscopy of an Integrin/Tie complex will identify the basis for growth factor receptor-integrin signal transduction.